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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
7/c = 42/72 // - 42/72
7/c-(42/72) = 0
7/c-7/12 = 0
7*c^-1 = 7/12 // : 7
c^-1 = 1/12
-1 < 0
1/(c^1) = 1/12 // * c^1
1 = 1/12*c^1 // : 1/12
12 = c^1
c = 12
c = 12
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